net/http: document that after Hijack, Request.Body is invalid
net/http: document that after Hijack, Request.Body is invalid

We can make it panic with a more explicit and readable error message
during Go 1.10, but document it for now. This has always been the
case; it's not a new rule.

Updates #20933

Change-Id: I53c1fefb47a8f4aae0bb32fa742afa3a2ed20e8a
Reviewed-on: https://go-review.googlesource.com/50634
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

josharian/go,Cofyc/go,golang/go,vsekhar/elastic-go,vsekhar/elastic-go,akutz/go,golang/go,cloudflare/go,vsekhar/elastic-go,codestation/go,nawawi/go,golang/go,cloudflare/go,vsekhar/elastic-go,akutz/go,cloudflare/go,polyverse-security/go,codestation/go,polyverse-security/go,codestation/go,josharian/go,golang/go,Cofyc/go,nawawi/go,akutz/go,vsekhar/elastic-go,CAFxX/go,Cofyc/go,Cofyc/go,vsekhar/elastic-go,josharian/go,josharian/go,vsekhar/elastic-go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,polyverse-security/go,akutz/go,CAFxX/go,josharian/go,codestation/go,CAFxX/go,josharian/go,polyverse-security/go,golang/go,nawawi/go,akutz/go,codestation/go,cloudflare/go,nawawi/go,akutz/go,golang/go,christopher-henderson/Go,Cofyc/go,codestation/go,polyverse-security/go,golang/go,cloudflare/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,josharian/go,polyverse-security/go,codestation/go,cloudflare/go,josharian/go,codestation/go,nawawi/go,CAFxX/go,polyverse-security/go,polyverse-security/go,akutz/go,cloudflare/go,Cofyc/go,CAFxX/go,golang/go,cloudflare/go,Cofyc/go,CAFxX/go,akutz/go